The present invention relates to drilling and completion techniques for hydrocarbon, geothermal, and analogous wells, and to stimulation techniques, or more generally to reservoir rock engineering techniques aimed at optimizing recovery of fluids contained in those reservoirs.
In the hydrocarbon well industry, a number of water-soluble polymers with gelling or viscosifying properties are used. The most frequently used polymers, such as vegetable gums, endow the solution with non-Newtonian properties with low viscosity at high shear rates and high viscosity at rest (a rheo-fluidifying fluid). For numerous applications, the opposite behaviour (rheo-viscosifying) is desirable.
Thus one known method for secondary recovery of hydrocarbons consists of injecting a flushing fluidxe2x80x94such as water to which polymers have been added to increase its viscosityxe2x80x94to displace the hydrocarbons towards the production well. A fluid in which the viscosity increases reversibly with shear forces could minimise viscous fingering problems and could render the flushing front uniform by preventing pockets of non flushed hydrocarbons from forming.
Rheo-viscosifying compositions can be formed by associating colloidal particles with polymers. The scientific publications listed below concern an association of a nanosilica (in precipitated form) with a poly(ethylene oxide) type homopolymer with a molecular mass in the range 4,000,000 to 5,000,000 g/mol.
Shear Induced Gelation Of Colloidal Dispersions. B. Cabane, K. Wong, P. Lindner, F. Lafuma in Journal of Rheology 41(3), May/June 1997
Rheological Behavior Of Moderately Concentrated Silica Suspensions In The Presence Of Absorbed Poly(Ethylene Oxide) S. F. Liu, F. Lafuma, R. Audebert in Colloid. Polym. Sci. 272, p 196-203, 1994
General Phase And Rheoloyical Behavior Of Silica/PEO/Water Systems S. F. Liu, V. Legrand, M. Gourmand, F. Lafuma, R. Audebert in Colloids and Surfaces 111, p 139-145, 1996.
With such systems, viscosification under shear is reversible, but such associations are quite fragile and very sensitive to the ionic strength of the medium and to the presence of surfactants, which destroy the rheo-viscosifying properties. High molecular weight poly(oxy)ethylenes are very expensive and they are manufactured in small quantities only, which is not favourable for industrial applications.
In Adsorption Of Polyacrylamide On Silica Particles And Its Effect On The Rheological Properties Of Suspension, Y. Otsubo and K. Umeya observed gelling phenomena in Aerosil silica/polyacrylamide systems (in a glycerine/water medium). Gelling in that case was irreversible above a critical shear rate. When the shear rate was reduced, the viscosity stayed at the level reached at a high shear rate and the behaviour thus became rheo-fluidifying.
There is thus a need for a fluid with reversible rheo-viscosifying properties which can be effectively exploited on an industrial scale.
The present invention provides a water-based fluid, primarily for drilling, completion, or any other intervention in a well such as a hydrocarbon well, in which the viscosity is controlled by a system constituted by precipitated silica nanoparticles (A) associated with a water-soluble copolymer (B) comprising one or more monomers (I) having little or no affinity for the silica and one or more co-monomers (II) which adsorb onto the silica.
The term xe2x80x9csilica nanoparticlesxe2x80x9d means particles with a diameter of less than 70 nanometers, preferably less than 40 nanometers and more preferably less than 10 nanometers.
The water-soluble copolymers of the invention must have a molecular mass of more than 700,000 g/mol, preferably more than 1,000,000 g/mol and more preferably more than 2,000,000 g/mol. The type (II) co-monomers must be in the minority and must be well dispersed in the copolymer which thus comprises at least 50%, and preferably at least 70%, of type (I) co-monomers, with a blocky statistical distribution, i.e., with a large number of very short sequences.
Examples of type (I) co-monomers are acrylamide, acrylic acid, methacrylic acid or any other acrylic acid derivatives substituted with an alkyl group in the xcex2 position, AMPS (2-acrylamido-2-methylpropanesulphonate), or sulphonated styrene. Acrylamide is preferred because of its moderate price, the readiness with which it produces high molecular masses, and its low interaction with the silica surface.
Type (II) co-monomers carry functions which are capable of adsorbing onto the silica and thus contain one or more heteroatoms which are well provided with available electron pair(s). Examples are vinylpyrrolidone, vinylimidazole (pH greater than 6), vinylpyridine, alkylvinylethers, N-vinylacetamide, and hydroxyethylmethacrylate and alkyl derivatives of these compounds. Two (or more) type (II) co-monomers with very different affinities for the silica surface can be incorporated into the copolymer so as to vary the effects on the rheology of the solution when shear is applied to that solution.